Ball grid array (BGA) packages, which connect to printed circuit boards (PCBs) using balls instead of pins or leads, are an alternative to existing peripheral-leaded surface mounted packages which require fine lead pitch for high I/O count packages.
For low I/O count devices (30-150 pins), fine pitch leaded packages offer body sizes and profiles that are comparable to currently offered BGA packages. However, BGAs are superior to fine pitch leaded packages for high pin count devices (208 pins or more). Additionally, the assembly process margins and yields with BGAs are substantially superior to fine pitch leaded packages.
BGA packages offer other advantages over the peripheral-leaded counterparts. For example, a BGA package has a large ball (i.e., lead) pitch of 1.27-1.5 mm as opposed to state of the art 0.3 mm lead pitch for a peripheral leaded QFP, (Quad Flat Pack). This large pitch enables good adaptability to existing SMT (Surface Mount Technology) manufacturing processes which use solder paste screen printing, component placement, then mass reflow.
For surface mount attachments of BGA packages, the solder deposit areas are larger and separated at larger pitches as compared to conventional SMT. This simplifies stencil solder printing. A larger and circular shaped opening in the stencil alleviates the problem often encountered in the conventional SMT of difficulties in forcing solder through narrow rectangular openings in the stencil.
The relatively large lead spacing of BGA packages also reduces placement tolerances that are critical in 0.5 or 0.4 mm pitch QFP packages. Thus, BGA packages are considered superior both for distribution of large I/Os (Inputs/outputs) on a small package footprint as well as ease in surface mount assembly.
Ease in assembly is exemplified by the fact that the surface tension of the molten solder ball provides a self-centering mechanism. A placement accuracy of 124 .mu.m between the land and the solder ball is often sufficient. This can be easily achieved by currently available mounting machines.
FIG. 1 illustrates a conventional ball grid array (BGA) package 10 known as "OMPAC" or Overmolded Plastic BGA package developed by Motorola. The overmolded plastic BGA package 10 has an organic substrate 15, such as BT resin based (Bismaleimide Triazine). A die 20 (i.e. an integrated circuit (IC) chip) is attached to the substrate 15 using a die attach adhesive 22.
The substrate 15 contains embedded multiple horizontal layers of metal conductors (not shown) connected by vertical conductors (not shown). The die 20 is positioned over the top surface 25 of the substrate 15. I/Os of the die 20 are connected to these embedded conductors by bonding wires 30. The bonding wires 30 are bonded to die bonding pads 32 on the die 20 and bonded to bond pad sites 35 on the top surface 25 of the substrate 15. The bonding wires 30 are attached as follows. A ball is formed at one end of a bonding wire 30 which is typically made of gold or aluminum. This ball is then attached by thermocompression jointly employing ultrasonic vibrations to the die bonding pad 32 on the die 20. A similar process is used to attach the bonding wires 30 to the bond pad sites 35 on the top surface 25 of the substrate 15. This process is done in the temperature range of 180.degree. C. to 200.degree. C., and typically at 180.degree. C.
The I/Os of the die 20 are routed to the bottom surface 40 of the substrate 15 through the metal conductors embedded in the substrate 15. In addition, through going vias 45, and thermal vias 50 used for heat transfer, may also be formed between the top and lower surfaces 25, 40 of the substrate 15. The through going vias 45 may serve to relieve mechanical stress.
Solder balls 55 are attached to land sites 60 at the bottom surface 40 of the substrate 15 and function as the I/O pins of the plastic BGA package 10. These sites 60 are connected to the die 20 via the metal conductors embedded in the substrate 15. The top surface 25 of the package 10 is encapsulated or overmolded by an epoxy molding compound 65 using a transfer molding process. In its final form, the package 10 is soldered to a printed circuit board (not shown).
FIG. 2 shows a bottom view of the package 10 shown in FIG. 1. As shown in FIG. 2, the bottom surface 40 of the substrate 15 has an array of solder balls 55 connected thereto. The package 10 is mounted to a PCB (not shown) using a vapor-phase or infrared solder reflow process. In either of these reflow processes, solder paste is deposited on the PCB and the array of solder balls 55 of the package 10 is positioned over the solder on the printed circuit board. A heat treatment is carried out so that the solder balls 55 reflow and attach the package 10 to the PCB.
Several other variations of conventional plastic BGA packages have been developed. These variations include different package configurations, encapsulation approaches, and substrate types.
In spite of the advantages of the conventional plastic BGA packages over peripheral-leaded surface mounted packages, the conventional plastic BGA packages have many problems, such as warpage, delamination and cracking.
In conventional plastic BGA packages, warpage of the substrate occurs after the overmolding process. This warpage poses a problem in SMT placement and attachment of the solder ball joints. Unlike the ceramic BGA packages, BT substrates of plastic BGA packages are vulnerable to warpage due to their low stiffness. Overmolded structures, such as the conventional BGA package 10 shown in FIG. 1, have a mismatch in their CTE (Coefficients of Thermal Expansion) between the epoxy molding compound 65 and the substrate 15. The substrate 15, as mentioned above, is BT resin based, which has a CTE (z-direction) of 4.7.times.10.sup.31 5 /.degree.C. The die or chip 20 is formed of silicon and has a CTE of 2.6.times.10.sup.-6 /.degree.C. This CTE or thermal mismatch creates interfacial stress as the overmolded structure is cooled to room temperature from elevated processing temperatures, such as during the die attach, wire bonding and overmolding processes.
The presence of the mold compound encapsulant 65 on only one side of the substrate 15 creates an unbalanced situation and leads to stress on the substrate 15. This causes warpage of about 140 microns for a 225 pins overmolded package. Warpage creates reliability concerns because it interferes with solder ball attachments and also impacts the integrity of other interfaces present in the package, such as the interface between the mold compound 65 and substrate 15.
In addition to warpage, delaminations occur between the overmold compound 65 and the substrate 15. The delaminations are caused by excessive interfacial and thermal stresses at the adjoining interface between the overmold compound 65 and the substrate 15. These residual thermal stresses may arise during both the manufacture and use of the package.
When molded, the entire package is at the mold temperature of approximately 180.degree. C. Upon cooling to room temperature, mismatches in the coefficients of thermal expansion (CTE) for the mold compound 65 and the substrate 15 result in residual stresses at the interface of these two materials. Furthermore, during soldering of the package 10 to the main circuit board of an electronic device (i.e., to a PCB), the soldering temperature of about 235.degree. C. creates additional thermal stresses at the interface between the substrate 15 and overmold compound 65. This is because the thermal expansion coefficients of the two materials are different. These interfacial stresses often lead to delaminations between the overmold compound 65 and the substrate 15. This is considered to be a primary failure mechanism in the overmolded BGA package 10.
Package cracking is another problem with the overmolded BGA package 10. Organic substrates and mold compounds absorb moisture from their environment, particularly for FR-4 type of substrates. When the packages are exposed to high temperatures, such as those involved in the solder reflow process (about 225.degree. C.), package cracking occurs due to thermal mismatch and the evaporation of expanded super steam under solder reflow conditions.
In addition, the thermal mismatch and the evaporating moisture weaken the interfacial bonds (between the mold compound 65 and the substrate 15 or between the die 20 and the substrate 15) leading to delaminations. The expanding moisture aggravates the delaminations and at the same time imposes high pressure at the interfacial bonds, causing package cracking. To alleviate this problem, the overmolded BGA package 10 is baked to dry (so that moisture is removed), and packed in hermetic bags. This is costly and not always a practical solution.
As shown in FIG. 1, the overmolded BGA package 10 has only one of the six faces of the substrate 15 encapsulated by the mold compound 65. The epoxy resins of the substrate 15 material is typically FR-4 or BT-glass. Both of these materials exhibit high moisture absorption. In addition, epoxy resins may contain high levels of chloride ion contamination. The absorbed moisture in conjunction with the chloride ions lead to galvanic corrosion of conductors and therefore reduce reliability.
Accordingly, it is an object of the invention to provide a BGA package and method for making a BGA package in which warpage, delamination and package cracking are reduced.